1. Field of the Invention
The present invention relates to a lever hoist with an overload preventing device.
2. Related Art
There has been a lever hoist in which rotation drive force is transmitted to a pressing member screwed in such a manner as to freely advance or retreat on a drive shaft by an operating lever, and a pressed member fixed to the drive shaft is pressed to be rotated by the pressing member via a reverse rotation preventing ring and a pair of friction members disposed at both surfaces of the ring.
Some types of overload preventing devices applied to the above-described lever hoist have conventionally used friction force, and have been configured such that a slide is generated between members so as to prevent any transmission of rotating force to the pressing member in the case where an overload is hoisted down.
However, in the conventional configuration, for example, a friction plate system, it has taken much time to adjust pressing force for the purpose of load limit setting since the pressing force to be exerted on a friction plate must be adjusted. Moreover, since it has been inevitable that the friction plate is worn out, a load limit has been varied as the friction plate is consumed. As measures against this, the pressing force has been required to be appropriately adjusted.
The present invention has been accomplished in view of the above-described circumstances. A primary object of the present invention is to provide a lever hoist with an overload preventing device, capable of easily setting a maximum hoisted load.
According to the present invention, a lever hoist with an overload preventing device comprises: a drive shaft for transmitting rotating force to a load sheave; a pressed member fixed to the drive shaft; a reverse rotation preventing ring which is rotated in only one direction with respect to the drive shaft; a pressing member screwed to the drive shaft in such a manner as to freely advance or retreat and disposed with the reverse rotation preventing ring held therebetween; a large-diameter boss projecting from the pressing member toward an axial tip; a small-diameter boss projecting from the large-diameter boss toward the axial tip and having a screw groove formed at the circumferential surface thereof; a rotation drive member rotatably fitted to the large-diameter boss of the pressing member in such a manner as to face the pressing member in the axial direction and having a drive tooth on the outer periphery thereof; a plurality of locking teeth, each having a triangular cross section, which are formed at respective circumferential surfaces facing each other, of the pressing member and the rotation drive member, so as to mesh with each other; a moderately inclined surface defining the locking tooth, for pressing the pressing member when the rotation drive member is rotated in a hoisting-up direction; a sharply inclined surface defining the locking tooth, for pressing the pressing member when the rotation drive member is rotated in a hoisting-down direction; an engaging portion formed at the projecting portion of the large-diameter boss projecting toward the tip beyond the rotation drive member when the rotation drive member is fitted to the large-diameter boss; a rotation limiting member having an outer diameter greater than the inner diameter of the rotation drive member, for restricting, at the outer peripheral edge thereof, movement toward the axial tip of the rotation drive member, and including, at the inner circumference thereof, an engaged portion to be engaged with the engaging portion at the tip of the large-diameter boss so as to inhibit relative rotation in a circumferential direction; urging means fitted to the small-diameter boss nearer the axial tip than the rotation limiting member, and having urging force such that it cannot ride over the sharply inclined surface of the locking tooth when the rotation drive member is rotated in the hoisting-down direction during a hoisting-down operation while it can ride over the moderately inclined surface of the locking tooth when an overload is exerted on the rotation drive member during a hoisting-up operation; and a nut screwed into the small-diameter boss, for positioning the urging means with respect to the pressing member.